A misfire condition (missed internal combustion) of an internal combustion engine, is generally due to the fact that fuel is not injected into the cylinder due to malfunctioning of the injector or the air/fuel mixture does not ignites because of malfunctioning of spark plugs, in spark ignition engines. Besides these misfire conditions, an incomplete (partial) combustion or lack of combustion may take place even for other causes, such as when the air/fuel mixture is excessively lean, or when the air/fuel mixture is excessively diluted with large amounts of exhaust gases.
These malfunctioning conditions, that foster misfire or partial combustion, generally occur when the engine is at idle and during fast changes of regime. For example, when the engine is decelerating, the injected fuel is diluted by exhaust gas of the previous cycle and the probability of occurrence of a misfire condition increases.
Notably, in spark ignition engines, to reduce the maximum firing temperature for reducing the formation of nitride oxides, often the expedient used is to dilute the injection mixture with aspired exhaust gas. This common technique, called Exhaust Gas Recirculation (or more briefly EGR), depending on the value of certain parameter of the engine (load, engine speed and the like) actuates a variable opening of an exhaust gas recirculation valve, and, thus, causes a different dilution of the air/fuel mixture. In particular conditions, an excessive dilution of the injection mixture may favor the occurrence of partial combustion or misfire.
Therefore, with high percentages of recirculated gases (high percentages of EGR), partial combustions and misfires are likely to occur more often.
It is also possible to have misfire or partial combustion of the air/fuel mixture because of faulty functioning of the lambda sensor and/or of the associated feedback control of the air/fuel ratio of the engine. There may be an excessively lean mixture with a consequent greater probability of misfires and partial combustions.
These phenomena may significantly limit the performance of the engine both in terms of power and of the amounts of emitted pollutants. Indeed, in the presence of misfire and/or partial combustion, the running of the engine becomes irregular, with undesired torque variations, loss of efficiency and a strong increase of the emissions of unburned hydrocarbons in the exhaust gases.
Misfires, in particular, besides reducing engine performance and increasing emissions of hydrocarbons, may damage the catalytic converter. Indeed, because of the high temperature in the catalytic converter (around 450° C.) following a misfire, the unburned mixture may ignite itself in the catalytic converter and damage it.
Identifying and analyzing the causes that may lead to misfires and partial combustion in spark ignition engines is important in engine control systems.
An effective control of partial combustion and an accurate identification and limitation of misfire events would permit improved performance of internal combustion engines and would reduce emissions of unburned hydrocarbons and prolong the life of the catalytic converter. Diagnosing misfire conditions is even required by certain regulations (EOBD, OBD II, etc.) relating to onboard diagnostic systems.
Generally, misfire conditions are diagnosed in many engines, by using speed sensors installed close to the crankshaft for sensing the occurrence of misfires by detecting variations of the angular speed of the crankshaft. Generally, a misfire condition is diagnosed by measuring the time duration of pulses of a phonic wheel of the engine: misfire events cause an abrupt deceleration of the rotation of the crankshaft, and as a consequence they abruptly increase the duration between two successive pulses of the phonic wheel.
This method of diagnosing a misfire condition has drawbacks due essentially to possible alterations of the motion unrelated to misfire conditions and limitations due to fabrication tolerances of the phonic wheel.
For example, a rough road on which a vehicle is running may influence the angular speed of its engine, and this is likely to cause spurious identification of misfires.
In order to address these problems, complex hardware and software are generally required for filtering data relating to the angular speed of the engine or as an alternative misfire diagnosis is momentarily disabled.
An alternative and more effective method of diagnosing misfires is based on the analysis of the pressure signal in the combustion chamber. In this case, the pressure signal is acquired by using a pressure sensor installed directly in the combustion chamber. Diagnosing misfire conditions with this technique has innumerable advantages with respect to other techniques commonly implemented in the ECU of engines and based substantially on the analysis of the angular speed of the engine.
One of the most important advantages of exploiting a pressure signal compared to analyzing the angular speed of the engine, is that eventual disturbances of the motion (torsional oscillations of the drive shaft and of the transmission gear, irregularities of the road, and so on) and eventual errors introduced by the phonic wheel do not affect the diagnosis of a misfire condition.
The indicated mean effective pressure (or more briefly IMEP) is the parameter of the pressure cycle that is more frequently used for identifying misfire phenomena. In a cycle in which a misfire takes place, the IMEP is negative (and also the torque). Unfortunately, calculation of this parameter is onerous because the IMEP is proportional to the integral along the whole pressure-volume cycle of the pressure with respect to the volume.